This invention concerns retainer arrangements for air (or other gas) filters which typically are constructed with a light weight casing made from a material such as thin paperboard or plastic into which is received an air filter element comprised of pleated filter paper or another type of filter element.
The casing includes a self-supporting U-shaped outer border which receives and encloses the outer perimeter of the air filter element.
A filter holder made of sheet metal or other sturdy material is configured to receive the air filter, and is mounted in the installation in which the air filter is to be used. The holder has a wall structure which is sized and matched to the filter to slidably receive the filter therein, with a usual perimeter clearance of ½ to ¾ inches, producing a space around the perimeter of a centered filter of ¼ to ⅜ inch. The filter is fit within the holder side structure with the case border portion seated on a holder flange projecting inwardly from the inner side of the bottom of the holder wall structure. When in service, the filter is lightly pressed against the flange by the resistance to air flow through the air filter element unless there is a reverse flow through the filter.
Leakage of air flow around the border of the air filter is sometimes attempted to be minimized by retainer arrangements urging the air filter against the holder flange, and a gasket mounted to either the air filter border or the holder flange is also sometimes included.
As noted, resistance to air flow through the pleated air filter element creates a force which normally at least lightly urges the air filter border against the holder flange, but installation where flow is in reverse direction would have the opposite result and increases the need to separately generate sealing pressure to maintain the filter firmly against the holder flange.
The air flow also tends to distort the filter by bulging the same, which is resisted by the filter casing which usually includes a cover sheet on each side with a pattern of flow openings in the cover sheets allowing air flow through the cover sheets.
These distorting forces may increase leakage around the border of the filter casing.
Separate stiff wire cross pieces are sometimes installed extending across the filter element to enable the filter to resist these distorting forces.
It is desirable that the air filter casing border be pressed against the holder flange to prevent significant volumes of unfiltered air flow from passing around the edges of the filter border, as such unfiltered flow could seriously degrade the performance of the filter. This becomes more acute where a reverse air flow is being filtered as mentioned above which creates forces tending to lift the filter casing out of engagement with the gasket or holder flange.
Furthermore, it is a delicate task to provide an arrangement for applying a sealing force against the typical lightweight filter casing particularly when the filter casing outer portions are relatively weak as when constructed of thin paperboard, since the filter border may be deformed significantly by the application of force to the border and allow bypass flow around the filter.
Accordingly, various arrangements have been used to urge the filter against the holder flange, but these have typically not generated an adequate sealing force urging the same against the gasket, due in some instances to the need to avoid excessively deforming the filter casing.
Air filters vary considerably in size and shape and have substantial differences in the sturdiness of the border portion of the filter case, and it is important to have a capability of varying the sealing forces so as to be practical for the sturdiness of a particular filter border.
Such attachments and retainer arrangements have typically involved the use of small removable hardware parts which are tricky to install and which can easily be separated from the holder and lost.
More importantly, adequate increases in sealing pressure typically have not yet been achieved by those arrangements.
It is also quite desirable that any such arrangement not require complicated or numerous steps to install or remove the filters, as this could become a maintenance burden, particularly for multiple filter installations.
Accordingly, it is an object of the present invention to provide a filter retainer arrangement which is capable of safely applying moderate but effective levels of sealing forces to a filter border without damaging the filter, but can also be conveniently increased when used with sturdier filter casings.
It is a further object of the invention to provide an easily installed filter retainer arrangement which does not involve the use of small, detachable hardware components which are apt to be lost.
Since many different sizes and configurations and operating conditions of filter installations may be encountered in the field, it is a further object of the invention to provide a filter retainer arrangement which can be easily adapted to variously configured and sized filters.